Process and device for controlling coating substance in a die caster

ABSTRACT

An installation has a fixed die portion and a moving die portion, ejector rods under the control of an actuator fed with control fluid, and a spray device for spraying a coating substance. A monitoring apparatus comprises a pressure sensor for continuously measuring the pressure of a control fluid, a device for measuring and recording the pressure as a function of time, a device for comparing the recorded measurement with a reference measurement, and a device for modifying the spraying of the coating substance as a function of the comparison.

BACKGROUND OF THE INVENTION

The present invention relates to a method and to apparatus formonitoring coating means in an installation for die-casting metals ormetal alloys.

More precisely, in an installation for die-casting metals or metalalloys, the present invention relates to apparatus for monitoring, i.e.optimizing, the spreading of a coating liquid on the inside faces of thedie used for performing the casting operation.

With reference initially to accompanying FIGS. 1a to 1d, an installationof known type is described for injection casting pieces made of metalalloy. The installation essentially comprises a die 12 made up of twoportions, respectively a moving portion 14 and a fixed portion 16, withportions of the inside faces thereof respectively referenced 14a and 16adefining the cavity 18 of the die. When the die is closed, the alloy tobe cast is placed in a container 20 and is injected under pressure intothe cavity 18 by a system of pistons 22. This is shown in FIGS. 1a and1b.

At the end of the casting operation, the moving portion 14 of the die ismoved away. To enable the casting 24 to be extracted from the portion 14of the die, extraction rods such as 26 are slidably mounted through theportion 14 of the die. These rods are operated simultaneously by anactuator that is not shown in the figures.

In addition, in order to avoid adhesion phenomena between the castingand the two portions 14 and 16 of the die, prior to closing the twoportions of the die and injecting the metal alloy, a coating operationis performed by means of a coating device 28 which serves to spray theinside faces 14a and 16a with a coating liquid that forms a release filmon said faces. The device can be stationary during the spray step or itmay be moved back and forth. It carries a certain number of spraynozzles. As is well known, there exist various coating substances, inparticular coating substances based on solvents or coating substancesthat can be diluted in water. These coating substances are themselveswell known. It will be understood that the problem is to match theconcentration of coating substance in the solvent, whether water or oil,with the quantity of substance that needs to be sprayed prior to eachcasting operation. It is of great importance to optimize the coatingoperation since it serves to reduce the force that needs to be exertedby the ejector 26 to expel the casting from the die. It will beunderstood that if excessive stress is applied, then the ejectors or thecastings being ejected run the risk of being damaged, which constitutesone of the major obstacles to automating die-casting.

It will therefore be understood that it is very important to be able tomonitor the effectiveness of the coating operation on a continuous basisand to be able to take action quickly as soon as the coating parametersare no longer satisfactory.

Another object of the invention is to provide monitoring apparatus thatmakes it possible automatically to modify the composition or flow rateof the coating substance as a function of the actual casting conditions.

Yet another object of the invention is to provide apparatus that cantrigger an alarm if the conditions under which a casting is ejected moveoutside normal operating conditions.

SUMMARY OF THE INVENTION

To achieve this object, the invention provides apparatus for monitoringcoating means in an installation for die-casting metals or metal alloys,said installation comprising a fixed die portion and a moving dieportion together defining a die cavity, ejector means for ejecting thecasting, the ejector means comprising ejector rods whose movements arecontrolled by an actuator fed with control liquid, and a spray devicefor spraying coating substance on the inside faces of thecavity-defining die portions, and means for feeding said strip withcoating substances, the apparatus being characterized in that itcomprises a single pressure sensor for continuously measuring thepressure of the control fluid in said actuator, means for recording saidpressure as a function of time during the casting ejection cycle, andmeans for comparing said recorded curve with a reference curve includingmeans for detecting a high pressure plateau (B) in the recorded curveand means for comparing the value of the pressure of said plateau with aminimum value (Pm) and with a maximum value (PM), and means formodifying said spraying of the coating substance as a function of theresults of said comparison.

It will be understood that the principle of the invention is to detectthe force an actuator needs to exert to proceed with the operation ofejecting the casting. The inventors have shown that the quality and theeffectiveness of the coating operation are inversely proportional to theforce which the ejection rods need to exert to eject the casting fromthe cavity of the die. It will be understood that by recordingvariations of pressure in this way, it is possible to act on the way thecoating device is fed in order to optimize the effects of the coating.

In a preferred implementation, the monitoring apparatus furthercomprises means for comparing the measured pressure curve with a maximumpressure and means for triggering an alarm if the measured pressureexceeds the maximum pressure.

It will be understood that this makes it possible to operate the castinginstallation without continuous human surveillance, since the alarmsystem informs the person responsible for surveillance when operatingconditions become unacceptable. Naturally, in a variant, the alarmsignal can automatically cause the die-casting installation to bestopped.

Also preferably, the apparatus further comprises means for modifying thefeed of coating substance to the spray device as a function of thecomparison between the measured pressure curve and the reference curve.

In this improved embodiment, it will be understood that the coatingparameters are adapted automatically as a function of the differencewhich exists between the measured curve of pressure in the ejectorcontrol actuator and the optimum pressure curve.

The invention also provides a method of monitoring the coating means,which method is characterized in that throughout the duration of anejection cycle, the pressure of the ejector actuator control fluid ismeasured, said measurements are recorded, and the coating parameters arevaried to optimize the measured curve of said pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention appearbetter on reading the following description of various embodiments ofthe invention given as non-limiting examples. The description refers tothe accompanying figures, in which:

FIGS. 1a to 1d, described above, illustrate a known type of die-castinginstallation;

FIG. 2a shows a casting installation provided with apparatusconstituting a first embodiment of the invention for monitoring thecoating means;

FIG. 2b shows one example of a curve of variation in the pressure of thecontrol fluid in the ejector control actuator; and

FIG. 3 shows a second embodiment of the monitoring apparatus enablingthe coating parameters to be adapted automatically.

DESCRIPTION OF THE INVENTION

With reference initially to FIG. 2a, a first embodiment of apparatus formonitoring the coating system is described. In this figure, there can beseen the two portions 14 and 16 of the die, together with a casting 24.There can also be seen the ejector rods 26 which are mechanicallyconnected to the rod 30 of a control actuator 32. The actuator 32 is fedwith control fluid via inlets 34 and 36. According to the invention, apressure sensor 38 is permanently mounted on the actuator 32 formeasuring the pressure that obtains within the control cylinder. Thesensor 38 may be of any appropriate type and it delivers an analogsignal which is optionally converted into digital data by a converter40. A processor circuit 42 serves to record and store the variouspressure values as a function of time throughout an ejection cycle. Thecorresponding curve can be displayed on a VDU such as 44. The processorcircuit 42 also includes comparator means that are described below. FIG.2a also shows the movable coating device 28 which is fed from a tank ofcoating liquid 46 via one or more control valves 48.

FIG. 2b shows the pressure P measured by the sensor 38 as a function oftime t. The pressure curve has a first pressure peak A which correspondsto control oil reaching the actuator. This peak is not representative ofejection forces.

Thereafter there is a rise in pressure with a high plateau B whichcorresponds to the casting being ejected from the moving portion of thedie under the effect of the ejector. The plateau B is of very shortduration (30 milliseconds (ms) to 50 ms) and corresponds to rupturing ofthe coating film and of the microbonds between the inside face of thedie and the casting. The curve also includes a third plateau C atconstant pressure but of lower value which corresponds to the end of theejection stroke. It has been discovered that the parameter which is themost meaningful and the most sensitive to variations in coatingcondition is the maximum pressure value in the plateau B. According toan important characteristic of the invention, it is thus the pressurevalue which corresponds to the plateau B which is retained and used asthe sole meaningful parameter concerning the ejection force and which istherefore used to optimize coating conditions.

It should be recalled that there are numerous coating parameters. Theeffectiveness of the coating stage and in particular the quality of thefilm of lubricant deposited on the inside faces of the die is responsiveto numerous parameters which vary from one run to another over a fairlywide range depending on the alloy under consideration and on the shapeof the casting to be made. Amongst these parameters, the following maybe mentioned:

the temperature of the die (170° C. to 350° C.);

the type of coating substance (ratio of wax over silicone, mineral orsynthetic oil);

the duration of spraying by means of the coating spray device (2 seconds(s) to 20 s);

the concentration of pure substance (0.5% to 5%);

the pressure of the air used for performing the spraying (3 bars to 6bars);

the pressure of the substance (3 bars to 6 bars);

the spraying distance (100 mm to 400 mm);

the flow rate of the substance through a spray nozzle (10 cm³ /s to 30cm³ /s);

the number of nozzles (4 nozzles to 20 nozzles or even more); and

the mode of spraying (strip stationary or moving back and forth).

In addition, numerous synergies exist between the various above-definedparameters that make a priori adjustments for optimizing the coatingstage difficult to perform. In the prior art, the beginning of any newproduction run has therefore required a prior setting-up stage ofgreater or lesser duration during which some or all of theabove-mentioned parameters have needed to be adjusted in order to obtainproper ejection. It will be understood that because of the monitoringapparatus, the various parameters can be optimized by performing varioustests prior to starting production of the castings. For each parameterthat is varied, the invention makes it possible to determine an optimumvalue of the parameter by selecting the value of the parameter that isdetermined as corresponding to the lowest possible pressure P for theplateau B.

During this preliminary stage, it is therefore possible to use theapparatus to adjust the various parameters which thus correspond to anoptimum pressure curve or more precisely to a maximum value for theplateau pressure B that is as low as possible.

During normal operation of the die, the monitoring apparatus thus makesit possible on each operation to verify whether the value of the maximumpressure is increasing relative to a previously-defined optimum value.Under such circumstances, it becomes necessary to modify the coatingparameters, as mentioned above.

Also, in a preferred implementation of the invention, the processordevice may continuously compare the maximum pressure corresponding tothe plateau B with a minimum acceptable pressure value Pm and with amaximum acceptable pressure value PM. If the measured value for themaximum pressure moves outside the pressure range Pm to PM, then theprocessor circuit 42 triggers an alarm circuit 50 which serves todisplay a visible alarm on the VDU 44 or to generate an audible alarm52. It will be understood that in this embodiment, the monitoringapparatus also makes it possible to provide surveillance over effectiveoperation of the system for ejecting from the die.

Reference is now made to FIG. 3 while describing a second embodiment ofthe monitoring apparatus in which a servo-control system is provided foradjusting the parameters of the coating liquid as a function of theinformation given by the pressure sensor. In this embodiment, theprocessor circuit 42 includes means for comparing the maximum pressureof the control liquid that corresponds to the plateau B with a referencevalue. If a difference exceeding a predetermined value is detected, thenthe processor circuit 42 activates a control circuit 60 which enablesvarious parameters governing the spraying of the coating liquid to bemodified. For example, the control circuit 60 controls the valve 48 foradjusting feed to the coating spray device 28 and, for example, a feedduct 62 for feeding solvent into the coating substance receptacle 46. Byautomatically controlling changes to the flow rate of the coating liquidin this way and also controlling spraying duration and the content ofconcentrated coating substance in the solvent, it is possible to modifyautomatically the parameters so as to return to optimum coatingconditions without any manual intervention being necessary. The coatingstage is thus servo-controlled.

I claim:
 1. In an installation for producing castings by die-castingmetals or metal alloys comprising a fixed die portion and a moving dieportion together defining a die cavity, an ejector for ejecting acasting, the ejector comprising ejector rods whose movements arecontrolled by an actuator fed with control liquid, and a spray devicefor spraying a coating substance on the inside faces of thecavity-defining die portions, and means for feeding said device with thecoating substance, an apparatus comprising a single pressure sensor forcontinuously measuring the pressure of the control liquid in saidactuator, means for recording a measurement of said pressure as afunction of time during a casting ejection cycle, means for recording acurve representative of the recorded pressure as a function of time,means for providing a reference curve, and means for comparing saidrecorded measurement with a reference measurement including means fordetecting a high pressure plateau (B) in the recorded measurement andmeans for comparing the value of the pressure of said plateau with aminimum value (Pm) and with a maximum value (PM), and means formodifying said spraying of the coating substance as a function of theresults of said comparison.
 2. An installation according to claim 1,having means responsive to said comparison to trigger an alarm if themeasured pressure does not lie between the values (Pm) and (PM).
 3. Aninstallation according to claim 1, having means for modifying the feedof coating substance to the spray device as a function of the comparisonbetween the recorded pressure curve and the reference pressure curve. 4.An installation according to claim 2, having means for modifying thefeed of coating substance to the spray device as a function of thecomparison between the pressure measurement and the reference pressuremeasurement.
 5. A method of controlling coating substance in theinstallation of claim 1, the method comprising continuously measuringthe pressure of the control liquid in said actuator, recording ameasurement of said pressure as a function of time during a castingejection cycle, recording a curve representative of the recordedpressure as a function of time, providing a reference curve, comparingsaid recorded measurement with a reference measurement includingdetecting a pressure plateau in the recorded measurement and comparingthe value of the pressure of said plateau with a minimum value and witha maximum value, and modifying said spraying of the coating substance asa function of the results of said comparison.